The information recording apparatus of this kind is used to record an audio signal on a sound track of, for example, a cinema film. As shown in FIG. 1, an optical image 1 having a triangular cross-section is irradiated through a light irradiating apparatus that includes a galvanometer on a linear-shaped slit 2 and the galvanometer is driven by an electrical audio signal to vibrate the optical image 1 in the direction shown by an arrow 3, whereby a scanning optical beam 4 of which the width is modulated in response to the audio signal is delivered via the slit 2. As shown in FIG. 2, when a cinema film is transported, this scanning optical beam 4 exposes and scans a sound track 5 to record on a sound track 5 an exposed locus 6 whose exposed area is changed with a central line X1 of the sound track 5 as the center.
The film thus exposed is chemically fixed with the result that the exposed locus 6 forms a transparent portion 7A and that the boundary between the transparent portion 7A and an opaque portion 7B draws a recording waveform 8 corresponding to the change of the audio signal.
By the way, when an information is recorded as described above, if the average level positions of the recording waveform 8 are fixed to the positions of, for example, lines X21, X22 in FIG. 2 and the recording waveform 8 is recorded with the average level positions X21 and X22 as the center even when the level of the audio signal is large (FIG. 3) or small (FIG. 4). Because the area of the transparent portion 7A is increased when the level of the audio signal is small, there occurs a problem that offensive noise based on scratches on the film and dust adhered to the film is mixed into the reproduced signal as a background noise. To solve this problem, a prior art background noise reduction circuit 11 shown in FIG. 7 is employed, in which the average level positions X21 and X22 of the recording waveform 8 are changed in response to the level of the audio signal so that when the signal level of the audio signal is small, the average level positions are made closer to the central line X1 as shown in FIG. 5, while when the signal level of the audio signal is increased, they are made distant from the central line X1 as shown in FIG. 6.
The background noise reduction circuit 11 supplies an input audio signal S1 through an input amplifier circuit 12 and a transformer 13 to a full wave rectifying circuit 14. A rectified output S2 therefrom is smoothed by a smoothing circuit 15, that generates a level detecting signal S3 corresponding to the signal level of the input audio signal S1 at the output terminal thereof. This level detecting signal S3 is supplied through an inverting amplifying circuit 16 to an adding circuit 17 in which it is added to the input audio signal S1. The resultant added output is delivered to the galvanometer as an average level position signal S4.
In the circuit arrangement shown in FIG. 7, if the signal level of the input audio signal S1 is decreased, the level detecting signal S3 is also decreased, increasing the average level position signal S4. Thus, the average level positions X21 and X22 of the recording waveform 8 on the sound track 5 are changed so as to approach the central line X1 as shown in FIG. 5. Conversely, if the signal level of the input audio signal S1 is increased, the level detecting signal S3 is increased, decreasing the average level position signal S4. Thus, the average level positions X21 and X22 of the recording waveform 8 on the sound track 5 are changed so as to come away from the central line X1 as shown in FIG. 6.
Consequently, when the input audio signal S1 as shown in FIG. 8A arrives, this signal is full-wave rectified by the full wave rectifying circuit 14 of the ground noise reduction circuit 11 and then smoothed by the smoothing circuit 15, resulting in the level detecting signal S3 (FIG. 8B) whose signal level is changed with a delay of substantially 20 [m sec] relative to the signal level change of the audio signal S1. Thus, in accordance with the change of the signal level of the audio signal S1, the average level positions X21 and X22 of the recording waveform 8 on the sound track 5 are corrected so that when the signal level is small, the exposed area of the exposed locus 6 is reduced, thus reducing the occurrence of the background noise in advance. The prior art circuit arrangement shown in FIG. 7, however, has the following defects and hence does not yet function satisfactorily as a background noise reduction circuit 11.
The first defect is such one that as to the the waveform of the level detecting signal S3 derived from the smoothing circuit 15, when the signal level of the input audio signal S1 is rapidly changed, the waveform of the level detecting signal S3 is not changed smoothly in correspondence therewith and hence there appears a sharp inflection point portion K1. When the level detecting signal S3 is changed rapidly as mentioned above, the amplitude of the recording waveform 8 on the sound track 5 is rapidly moved and at the same time, the average level positions X21 and X22 are rapidly moved so as to come away from the central line X2. Therefore, the recording waveform 8 is distorted. If this distorted waveform is reproduced, pop noise heard as "buchi" or "pu" appears in the reproduced sound. By the way, when the sound begins to emanate, since the signal level of the input audio signal S1 rapidly rises up frequently, such pop noise frequently occurs and hence the tone quality is deteriorated.
The second defect is such one that when the signal level of the input audio signal S1 is changed, the change of the level detecting signal S3 derived from the smoothing circuit 15 in response to such change is with a delay of about 20 msec. Accordingly, when the signal level of the input audio signal S3 is increased, the movement of the average level positions X21 and X22 of the recording waveform 8 is delayed by the response delay time, resulting in an over-modulation state. As a result, the recording waveform 8 corresponding to the rising-up portion of the signal level of the input audio signal S1 is distorted so that a reproduced signal faithful to the input audio signal S1 can not be obtained.
Further, the third defect is such that the frequency characteristic of the circuit arrangement shown in FIG. 7 presents, when a white noise is applied to the input terminal thereof, such a characteristic that the frequency component of 20 to 60 Hz is attenuated by about 20 to 40 dB and then remained. Accordingly, there is brought about such a result that this low frequency component is superimposed upon the audio signal, recorded and reproduced and thereby the reproduced signal is distorted. By the way, in order to avoid this defect, it may be considered that the cut-off frequency of the smoothing circuit 15 is selected to be low. If this proposal is realized, the response characteristic is delayed more with the result that the aforesaid second defect is fostered. This is not desirable.
In view of the above mentioned point, the present invention is intended to provide an information recording apparatus which effectively removes the sharp inflection point of the waveform of the level detecting signal without causing the over-modulation due to the delay of the response and hence which is free from the above defect.